1. Field of the Invention
The present invention relates to a rare earth magnet having high strength and high electrical resistance.
Priority is claimed on Japanese Patent Application Nos. 2005-170475, filed on Jun. 10, 2005, 2005-170476, filed on Jun. 10, 2005, and 2005-170477, filed on Jun. 10, 2005, the contents of which are incorporated herein by reference.
2. Description of Related Art
An R—Fe—B-based rare earth magnet, where R represents one or more kind of rare earth element including Y (this applies throughout this application), is known to have such a composition that contains R, Fe and B as basic components with Co and/or M (M represents one or more kind selected from among Ga, Zr, Nb, Mo, Hf, Ta, W, Ni, Al, Ti, V, Cu, Cr, Ge, C and Si; this applies throughout this application) added as required, specifically, 5 to 20% of R, 0 to 50% of Co, 3 to 20% of B and 0 to 5% of M are contained (% refers to atomic %, which applies throughout this application), with the balance consisting of Fe and inevitable impurities.
It is known that the R—Fe—B-based rare earth magnet can be manufactured by subjecting an R—Fe—B-based rare earth magnet powder to hot pressing, hot isostatic pressing or the like. One of methods of manufacturing the R—Fe—B-based rare earth magnet powder is such that an R—Fe—B-based rare earth magnet alloy material that has been subjected to hydrogen absorption treatment is heated to a temperature in a range from 500 to 1000° C. and kept at this temperature in hydrogen atmosphere of pressure from 10 to 1000 kPa so as to carry out hydrogen absorption and decomposition treatment in which the R—Fe—B-based rare earth magnet alloy material is caused to absorb hydrogen and decompose through phase transition, followed by dehydrogenation of the R—Fe—B-based rare earth magnet alloy material by holding the R—Fe—B-based rare earth magnet alloy material in vacuum at a temperature in a range from 500 to 1000° C. It is known that the R—Fe—B-based rare earth magnet powder thus obtained has recrystallization texture consisting of adjoining recrystallized grains that are constituted from R2Fe14B type intermetallic compound phase that has substantially tetragonal structure as the main phase, and the recrystallization texture has the fundamental structure of magnetically anisotropic HDDR magnetic powder in which the fundamental structure has such a constitution that 50% by volume or more of the recrystallized grains are those which have such a shape as the ratio b/a of the least grain size a and the largest grain size b of the recrystallized grains is less than 2, and average size of the recrystallized grains is in a range from 0.05 to 5 μm (Japanese Patent No. 2,376,642).
In recent years, automobiles are employing increasing numbers of electrically powered devices, while great efforts are being made in the development of electric vehicles. In line with these trends, research and development activities have been increasing for the development of compact and high performance electronic devices and motors based on permanent magnet, for onboard applications. Improvement in the performance of the compact and high performance electronic devices and motors based on permanent magnet inevitably requires it to use the R—Fe—B-based rare earth magnet that has high magnetic anisotropy. However, the ordinary R—Fe—B-based rare earth magnet is a metallic magnet and therefore has low electrical resistance which, when used in a motor, causes a large eddy current loss that decreases the efficiency of the motor through heat generation from the magnet and other factors. To avoid this problem, R—Fe—B-based rare earth magnets that have high electrical resistance have been developed. It has been proposed to make one of these R—Fe—B-based rare earth magnets that have high electrical resistance by forming an R oxide layer in the grain boundary of R—Fe—B-based rare earth magnet particles so that the R—Fe—B-based rare earth magnet particles are enclosed with the R oxide layer to make a structure (Japanese Unexamined Patent Application, First Publication No. 2004-31780 and Japanese Unexamined Patent Application, First Publication No. 2004-31781).
However, since the rare earth magnet of the prior art that has high electrical resistance has a structure such that the R oxide layer exists in the grain boundary of the R—Fe—B-based rare earth magnet particles, bonding strength between the R—Fe—B-based rare earth magnet particles is weak, and therefore, the rare earth magnet of the prior art that has high electrical resistance has the problem of insufficient mechanical strength.